The present invention relates to radiation therapy systems using ions, such as protons, for the treatment of cancer and the like and, in particular, to a system providing improved modulation of a ion beam.
External beam radiation therapy may treat a tumor within the patient by directing high-energy radiation in one or more beams toward the tumor. Recent advanced external beam radiation systems, for example, as manufactured by Tomotherapy, Inc., treat a tumor with multiple x-ray fan beams directed at the patient over an angular range of 360°. Each of the beams is comprised of individually modulated beamlets whose intensities can be controlled so that the combined effect of the beamlets, over the range of angles, allows an arbitrarily complex treatment area to be defined.
X-rays deposit energy in tissue along the entire path between the x-ray source and the exit point in the patient. While judicious selection of the angles and intensities of the x-ray beamlets can minimize radiation applied to healthy tissue outside of the tumor, inevitability of irradiating healthy tissue along the path to the tumor has suggested the use of ions such as protons as a substitute for x-ray radiation. Unlike x-rays, protons may be controlled to stop within the tissue, reducing or eliminating exit dose through healthy tissue on the far side of the tumor. Further, the dose deposited by a proton beam is not uniform along the entrance path of the beam, but rises substantially to a “Bragg peak” near a point where the proton beam stops within the tissue. The placement of Bragg peaks inside the tumor allows for improved sparing of normal tissue for proton treatments relative to x-ray treatments.
Unlike x-rays, protons may be controlled to stop within the tissue, eliminating exit dose through healthy tissue on the far side of the tumor. Further, the dose deposited by a proton beam is not uniform along the entrance path of the beam, but rises substantially at a “Bragg peak” near a point where a proton stops within the tissue. Proton therapy is described generally in U.S. Pat. No. 5,668,371 entitled “Method and Apparatus for Proton Therapy” issued Sep. 16, 1997, assigned to the assignee of the present invention and hereby incorporated by reference.
In distinction from x-rays, with protons it is possible to separately control intensity (i.e., the average number of protons per time over an area) and energy (i.e., the speed of the protons). Control of the intensity of protons determines the dose delivered by the protons to the tissue whereas control of the energy of the protons determines the depth in the tissue at which the dose is concentrated. In the above reference patent application, the intensity of the protons within different “beamlets” of a fan beam are controlled by changing the time during which blocking shutters are placed in the path of each beamlet versus the time the blocking shutters are removed from the path of the beamlets. By “duty cycle” modulating, the shutter intensity variations may be obtained.
A similar approach may be adopted in proton therapy systems that use a steerable pencil beam (rather than a fan beam) of protons. In this case the “dwell time” of the pencil beam at a particular location before it is moved determines the intensity of protons delivered to that location.
Both of these approaches will be termed “time accumulation” approaches as they rely on changing the length of time the tissue is exposed (and thus the average intensity of the beam) to control the dose. A drawback to such time accumulation systems is that higher average intensities require correspondingly increased exposure times. As a practical matter this increases treatment times. Designing fast acting shutters or pencil beam scanning systems, that might offset these increased treatment times, can be difficult or expensive.